An example of a conventional optical disc drive will be described with reference to FIGS. 5 to 8. Such an optical disc drive is disclosed in Patent Document No. 1, for example.
First, look at FIG. 5. FIG. 5(a) is a cross-sectional view showing the configuration of a conventional optical disc drive and FIG. 5(b) shows one side of its light source and surrounding members. The side crosses the paper of FIG. 5(a) at right angles.
In the conventional optical disc drive shown in FIGS. 5(a) and 5(b), a laser beam 1a that has been emitted with a wavelength λ from a light source 1 such as a semiconductor laser diode, which is mounted on a photodetector substrate 9, is reflected by a reflective mirror 10, which is also arranged on the photodetector substrate 9, and then transformed by a collimator lens 4 into a parallel beam, of which the optical axis is defined by the line 7.
The parallel beam, of which the optical axis is defined by the line 7, is transmitted through a polarizing hologram substrate 2 and then transformed by a quarter-wave plate 3 from a linearly polarized light beam (which is either an S-wave or a P-wave) into an elliptically polarized light beam. Thereafter, the elliptically polarized light beam is condensed by an objective lens 5 with a focal length f to be converged on a signal plane 6a of an optical disc 6.
The light that has been reflected from the signal plane 6a is passed through the objective lens 5 and then transformed by the quarter-wave plate 3 into a linearly polarized light beam (which is either a P-wave or an S-wave) again. This linearly polarized light beam is then incident on the hologram plane 2a of the polarizing hologram substrate 2, diffracted, and then branched into first-order diffracted light 8′ and −first-order diffracted light 8″, which are symmetric to each other with respect to the optical axis 7 as the axis of symmetry. The branched light (diffracted light) beams are converged through the collimator lens 4 and then incident on the detector plane 9a of the photodetector substrate 9. The quarter-wave plate 3 is arranged on the polarizing hologram substrate 2 and moves synchronously with the objective lens 5.
The detector plane 9a is located substantially on the focal plane of the collimator lens 4 (i.e., at the virtual emission point of the light source 1). On the signal plane 6a of the optical disc 6, either guide grooves or pit sequences are arranged at a pitch  in the disc radial direction so as to run in the disc rotating direction.
In the space between the light source 1 and the collimator lens 4, the optical axis 7 is aligned with the center axis of the collimator lens 4 and the extension thereof. On the other hand, in the space between the collimator lens 4 and the signal plane 6a of the optical disc 6, the optical axis 7 is aligned with the center axis of the objective lens 5 and the extension thereof.
FIG. 6(a) shows the distribution of light on a plane that intersects with the optical axis 7 of the reflected light at right angles just before the reflected light reaches the hologram plane 2a in the optical disc drive described above. On the other hand, FIG. 6(b) shows the distribution of light intensity on a cross section that is defined in the disc radial direction and that corresponds to distribution of the reflected light shown in FIG. 6(a).
The x- and y-axes shown in FIG. 6(a) correspond to the disc radial direction and the rotating direction, respectively. The optical axis 7 passes the intersection O between the x- and y-axes.
In FIG. 6(a), when reflected from the signal plane 6a of the optical disc, the light beam is diffracted by the periodic structure with the pitch  on the signal plane 6a of the optical disc. In the reflected light that is about to enter the hologram plane 2a, zero-order diffracted light 11 partially overlaps with not only first-order diffracted light 11a but also −first-order diffracted light 11b. 
Each of diffracted light beams has its center shifted by a distance d in the disc radial direction (i.e., in the x-axis direction) and has a light distribution in which a portion of the light outside of the aperture 5a of the objective lens 5 is cut off in the area with a radius r that is greater than r0, where r0 is the aperture radius of the objective lens 5. However, the first-order diffracted light 11a has shifted in the positive direction along the x-axis, while the −first-order diffracted light 11b has shifted in the negative direction along the x-axis. And the magnitude d of shift may be given by the following Equation (1), where λ is the wavelength of the light emitted from the light source 1, f is the focal length of the objective lens 5 and is equal to r0/NA (where NA is the numerical aperture of the objective lens 5) and  is the pitch of the periodic structure on the signal plane 6a of the optical disc 6.d=fπ/  (1)
The phase relation between the intensities of the zero-order diffracted light and the ±first-order diffracted light is determined by the shape parameters (including the pitch, groove width and groove depth) of the guide grooves on the signal plane 6a and the location of the light beam spot with respect to the guide grooves (including the magnitude of off-track with respect to the center of the guide grooves and the magnitude of defocusing). The distribution of the reflected light, produced as a result of the mutual overlap and interference of these light beams, is also determined by the shape parameters of the guide grooves, the magnitude of off-track and the magnitude of defocusing.
The intensity distribution of the laser beam 1a emitted from the light source 1 is usually a Gaussian distribution and is adjusted such that when the objective lens 5 is located at its reference position, the center of the light intensity distribution is located at x=y=0 on the way toward the disc. The zero-order diffracted component of the reflected light has a distribution obtained by inverting that Gaussian distribution with respect to the optical axis. Also, when the magnitude of off-track is zero, the light distribution of the reflected light, produced by the overlap of diffracted light beams, often has a quasi-Gaussian distribution, too. Actually, however, there will be some adjustment errors and other errors, and therefore, the reflected light has an intensity distribution that is asymmetric with respect to the origin. And the curve 13 shown in FIG. 6(b) is drawn on that model. Furthermore, the real Gaussian distribution has such significant fluctuations that the light intensity may be rather low (or high) in some areas. The distribution of the reflected light may also fluctuate due to the presence of dust that has been deposited on the surface of a lens or the disc base member. The area 12 shown in FIG. 6(a) represents such a fluctuation and functions as an area with a lower intensity than its surrounding areas as represented by the curve 13a in FIG. 6(b). Also, if the objective lens 5 shifted in the radial direction of the optical disc while a tracking control is being performed on the optical disc, the light intensity distribution of the reflected light would be represented by the curve 13′ that has shifted in the opposite direction.
FIG. 7 shows how the light beam spot is divided on the hologram plane 2a. The hologram plane 2a does not necessarily have to intersect with the optical axis 7 at right angles but is supposed to do that in the following description. The x- and y-axes correspond to the disc radial direction and rotating direction, respectively, and the optical axis 7 passes the intersection O between these two axes.
As shown in FIG. 7, the hologram plane 2a is divided by division lines 15a, 15b and 15c into six areas 14a, 14a1, 14a2, 14b, 14b1 and 14b2. A portion of the light that has been reflected from the signal plane 6a of the optical disc and then incident on the hologram plane 2a is branched by these areas, thereby producing branched light beams. And the intensities of the branched light will be detected independently of each other by the photodetectors on the detection plane 9a. The intensities of those light beams detected will be identified by S14a, S14a1, S14a2, S14b, S14b1 and S14b2, respectively.
The division line 15a equally splits the zero-order diffracted light 11 into two in the disc rotating direction (i.e., along the y-axis), while the division lines 15b and 15c divide portions of the zero-order diffracted light 11 near the outer periphery in the disc radial direction (i.e., along the x-axis) so as to choose portions that are not overlapped by the ±first-order diffracted light 11a and 11b. The following two signals can be figured out based on the detection signals of the photodetectors.TE1=S14a−S14b  (2)TE2=S14a1+S14a2−(S14b1+S14b2)  (3)
The tracking error signal TE, representing the magnitude of error with respect to the guide groove (or pit sequence) on the signal plane 6a of the optical disc 6, may be calculated, using a coefficient value k, by the following Equation (4).TE=TE1−k×TE2  (4)
While the tracking control is performed on the guide grooves, the objective lens 5 shifts in the radial direction synchronously with the in-plane vibrations of the optical disc. As a result, the intensity distribution of the reflected light also shifts from that represented by the curve 13 to that represented by the curve 13′ as shown in FIG. 6(a). And TE1 and TE2 are also affected.
Generally speaking, the off-track of a light beam spot with respect to the guide grooves changes information about the phase relation between the zero-order diffracted light and the ±first-order diffracted light. That is why in an area where these diffracted light beams interfere with each other, the light intensity varies. Equation (2) covers the area where the zero-order diffracted light and the ±first-order diffracted light interfere with each other, while Equation (3) covers only the area with the zero-order diffracted light. For that reason, information about the off-track is included in TE1 but not in TE2. That is to say, only information about a lens shift is included in TE2. Consequently, if the coefficient value k is selected appropriately, a tracking error signal that is not affected by any lens shift can be generated by Equation (4).
FIG. 8 shows how the light beam spot may be divided on the hologram plane 2a in another conventional drive.
In this example, the x- and y-axes also correspond to the disc radial direction and rotating direction, respectively, and the optical axis 7 also passes the intersection O between these two axes. The hologram plane 2a is divided by division lines 15a, 15d and 15e into four areas 14a, 14a1, 14b and 14b1. A portion of the reflected light that has been incident on the hologram plane 2a is branched by these areas, and the intensities of the resultant branched light beams will be detected independently of each other by the photodetectors on the detection plane 9a. The intensities of those light beams detected will be identified by S14a, S14a1, S14b and S14b1, respectively. The division line 15a equally splits the zero-order diffracted light 11 into two in the disc rotating direction (i.e., along the y-axis), while the division lines 15d and 15e divide portions of the zero-order diffracted light 11 near the y-axis along the outer edge of the ±first-order diffracted light 11a and 11b so as to choose portions that are not overlapped by the ±first-order diffracted light 11a and 11b. 
The following two signals TE1 and TE2 can be calculated by Equation (2) and the following Equation (5) based on the detection signals of the photodetectors.TE2=S14a1−S14b1  (5)
The tracking error signal TE, representing the magnitude of error with respect to the guide groove (or pit sequence) on the signal plane 6a of the optical disc 6, may be calculated by Equation (4) using the coefficient value k. TE1 is also affected by a lens shift and an off-track as in the example shown in FIG. 7 but TE2 is affected only by a lens shift because it represents the area with only the zero-order diffracted light. That is why by selecting an appropriate coefficient value k, a tracking error signal that will not be affected by any lens shift can be figured out by Equation (4).                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 9-223321        